Epidemiologic and experimental evidence indicates a relationship between a high fat diet and breast cancer. The objective now is to determine if this relationship is useful in breast cancer prevention. A Phase III randomized controlled trial is proposed to evaluate the feasibility and preventive effect of a low fat diet (20% of calories) in women aged 45-69 at increased risk for developing breast cancer. After determining increased risk status and eligibility, 200 women will be randomly assigned either to the low fat diet intervention group or to the regular diet control group. Initially a feasibility trial will evaluate the continuous adherence of these women to a low fat diet through a multidisciplinary intervention program that will take place over a 10 month period. The low fat diet intervention consists of a 20% fat diet instruction, facilitated by group behavior modification sessions and an aerobic exercise program. The regular diet consists of a balanced diet based on the four basic food groups, with a normal level of dietary fat (about 40% of calories). Provision will be made for all women to attain the recommended daily dietary allowance of major micro- and macro-nutrients. Periodic assessments of dietary intake, compliance, body weight, and serum lipids will be made on both groups. If the results of the feasibility trial are satisfactory, then a full scale long-term low fat diet intervention trial will be undertaken, following any necessary modifications in the basic protocol.